Hypnos is the personification of sleep in Greek mythology, son of Nyx and brother to Moros and Thanatos. He lives in a cave with the entrance dotted by narcotic flowers. Hypnos is often depicted as a young man wrapped in poppy stock, a horn in his hand, and sometimes wings on his head.
Hypnos appears as the boss of the Hypnos Tower in the Snow Queen Quest. The Snow Queen granted Hypnos as a guardian to Kumi Hirose, which she used to put the school faculty and students to sleep so they could live their dreams forever.
Hypnos appears in the 33th and 34th chapters of the manga, with a role identical to his game counterpart. He was defeated by Maki Sonomura at the start of the 34th chapter with Tensen Nyannyan's Heat Wave spell.
Hypnos is Takaya Sakaki's artificially summoned Persona, born from the experiments performed by the Kirijo Group. Surprisingly enough, Takaya is able to summon Hypnos without the aid of an Evoker. Hypnos appears as a disfigured body held in the air by wings seemingly grafted into his spine.
It is possible to hack Hypnos for the Protagonist to use, but unlike party member Personas, he has no voice clip for Hypnos and the game will crash if attempting to use it. Most likely, this was just to test Hypnos would function like other Personas. There is a picture of Hypnos among the rest of the Personas, however, which is also likely for testing purposes.